1. Field of the Invention
The present invention relates in general to signal modulators and specifically to a method and apparatus for maintaining linear operation of a signal modulator.
2. Description of the Related Art
Communication systems use transmitters having signal modulators that modulate a desired signal onto a carrier signal before transmission. Optical communication systems typically employ high power optical transmitters. For example, cable television broadcast networks use high power optical transmitters to transport multiple video channels through fiber optics for signal distribution. These transmitters typically utilize electro-optic intensity modulators to modulate an optical signal before transmission.
Electro-optic modulators are constructed by placing metal electrodes above and below an optical waveguide formed in a crystal. Lithium niobate (LiNbO.sub.3) is typically used for the crystal. A desired signal is modulated onto the electrodes, and the electric field between the electrodes, in turn, modulates the crystal, changing its optical index of refraction in response to the input signal. As the refractive index changes, the light (typically from a continuous wave (CW) source laser), is modulated as it passes through the external modulator.
External modulators have a sinusoidal transfer function of modulation voltage input (across the electrodes) in relation to light modulation output. Proper operation of the modulator requires that a direct current (D.C.) bias voltage be applied across the electrodes of the modulator, and that the bias point must be maintained at the point of inflection in the sinusoidal transfer function, called the quadrature bias point.
However, the quadrature bias point is subject to drift due to temperature and to charges accumulated in the optical crystal over time. As the D.C. bias moves away from the quadrature bias point, the modulation transfer function causes significant second order intermodulation distortion in the transmitted signal. Therefore, for proper system operation, the bias point must be maintained at the quadrature point.
One attempt at maintaining the proper bias point includes injecting a radio frequency (RF) pilot tone (also referred to as a continuous wave (CW) pilot signal) into the modulator in addition to the modulation signals. A portion of the optical output from the external modulator is detected by an optical receiver. A harmonic signal of the injected pilot signal is monitored at the output of the receiver. The amplitude of the harmonic signal is a function of the difference between the quadrature point on the modulator's transfer function, and the actual D.C. bias point. When biased exactly at quadrature, the amplitude of the harmonic signal is suppressed. By monitoring the harmonic signal of the pilot signal, a feedback loop adjusts the bias voltage in order to maintain quadrature bias.
However, current approaches to implementing this bias control loop are unreliable. The dynamic range of the magnitude of the harmonic signal can exceed 100 dB. Since current approaches use an analog feedback loop, accurate detection of the harmonic signal is lacking. As the quadrature bias point is reached, the harmonic signal amplitude drops below the noise floor of the system. As a result, the loop can not maintain the bias point.
Therefore there exists a need for a method and apparatus for maintaining the optimum bias point of a signal modulator.